Facebook has a long way to go yet before it'll replace twitter, but I can see the appeal for some.
Facebook is working on incorporating the hashtag, one of Twitter's most iconic markers, into its service by using the symbol as a way to group conversations, said people familiar with the matter. It is unclear how far along Facebook's work on the hashtag is and the feature isn't likely to be introduced imminently, these people said.Report: Facebook Looks At Incorporating Hashtags [online.wsj.com]
It would be a good addition to their search. As much as I don't believe Facebook is a viable business model, I have to give them kudos for thinking of every possibility to improve their service or make more money.
I think this makes perfect sense from a FB point of view. There are users who pair up their FB and Twitter accounts so cross posting occurs. Due to this we already see the use of #tags on Facebook pages. It makes sense to make use of them.